Semiconductor devices using a compound semiconductor such as GaAs, for example, a field-effect type transistor (hereinafter, referred to as FET) is frequently used because it has an excellent high-frequency characteristic and is operated in a microwave band. In semiconductor devices of this type, a recess area with a vertical sidewall is prepared on the surface of a semiconductor layer. Then, an insulating film having a through hole with a vertical sidewall, through which the recess area is exposed, is formed on the semiconductor layer, and a gate electrode is formed in the recess area and the through hole.
However, along with the miniaturization of recent semiconductor devices, the aspect ratio of the recess area has increased. When a gate electrode is formed in the recess area with a large aspect ratio by vapor-depositing, it is difficult to vapor-deposit a metal on the vertical sidewall of the recess area, and it is difficult to form a gate electrode so that the entire recess area is filled with the gate electrode. If the recess area is not filled with the gate electrode, the sidewall of the recess area becomes exposed, and the surface becomes contaminated during each manufacturing postprocess. This is one of causes for the degraded performance of the semiconductor devices.
In addition, in the process of forming the gate electrode by vapor-depositing, the metal is not vapor-deposited on the sidewall of the through hole of the insulating film. Instead, when the metal is vapor-deposited, it extends over the periphery of the through hole onto the surface of the insulating film. Therefore, a space (void) is generated between the gate electrode and the sidewall of the through hole. If a void is generated in this manner, cracks are generated in the gate electrode in the process for forming the gate electrode. In the gate electrode in which the cracks have been generated, the resistance value rises from a prescribed value, leading to one of the causes for the degraded performance of the semiconductor devices.
An example of related art includes Patent Reference of JP-A-11-135477.